The present invention relates to a method for etching substrates and cleaning the etching chamber.
In the manufacture of integrated circuits, silicon dioxide, silicon nitride, polysilicon, metal silicide, and monocrystalline silicon on a substrate, are etched in predefined patterns to form gates, vias, contact holes, trenches, and/or interconnect lines. In the etching process, a patterned mask layer composed of oxide or nitride hard mask or photoresist, is formed on the substrate using conventional methods. The exposed portions of the substrate between the patterned mask are etched by capacitive or inductively coupled plasmas of etchant gases. During the etching processes, a thin etch residue deposits on the walls and other component surfaces inside the etching chamber. The composition of the etch residue depends, among other things, upon the composition of vaporized species of etchant process gas, the substrate material being etched, and the mask or resist layer applied on the substrate. For example, when tungsten silicide, polysilicon or other silicon-containing layers are etched, silicon-containing gaseous species are vaporized or sputtered from the substrate, and etching of metal layers results in vaporization of metal species. In addition, the resist or mask layer on the substrate is also partially vaporized by the etchant gas to form gaseous hydrocarbon or oxygen species. The vaporized or gaseous species in the chamber condense to form polymeric byproducts composed of hydrocarbon species from the resist; gaseous elements such as fluorine, chlorine, oxygen, or nitrogen; and elemental silicon or metal species depending on the composition of the substrate being etched. The etch byproducts deposit as thin layers of etch residue on the walls and components in the chamber. The composition of the etch residue layer typically varies considerably across the chamber surface depending upon the composition of the localized gaseous environment, the location of gas inlet and exhaust ports, and the chamber geometry.
The compositionally variant, non-homogeneous, etch residue layer formed on the etching chamber surfaces has to be periodically cleaned to prevent contamination of the substrate. Typically, after processing of about 25 wafers, an in-situ plasma xe2x80x9cdry-cleanxe2x80x9d process is performed in an empty etching chamber to clean the chamber. However, the energetic plasma species rapidly erode the chamber walls and chamber components, and it is expensive to often replace such parts and components. Also, erosion of the chamber surfaces can result in instability of the etching process from one wafer to another. The thin compositionally variant etch residue can also make it difficult to stop the in-situ plasma clean process upon removal of the thin layer of residue, resulting in erosion of the underlying chamber surfaces, and making it difficult to clean the hard residue off all the chamber surfaces. For example, the etch residue formed near the chamber inlet or exhaust often has a higher concentration of etchant gas species than that formed near the substrate which typically contains a higher concentration of resist, hard mask, or of the material being etched.
It is difficult to form a cleaning plasma that uniformly etches away the compositional variants of etch residue. Thus after cleaning of about 100 or 300 wafers, the etching chamber is opened to the atmosphere and cleaned in a xe2x80x9cwet-cleaningxe2x80x9d process, in which an operator uses an acid or solvent to scrub off and dissolve accumulated etch residue on the chamber walls. To provide consistent chamber surface properties, after the wet cleaning step, the chamber surfaces are xe2x80x9cseasonedxe2x80x9d by pumping down the chamber for an extended period of time, and thereafter, performing a series of runs of the etch process on dummy wafers. The internal chamber surfaces should have consistent chemical surfaces, i.e., surfaces having little or no variations in the concentration, type, or functionality of surface chemical groups; otherwise, the etching processes performed in the chamber produce widely varying etching properties from one substrate to another. In the pump-down process, the chamber is maintained in a high vacuum environment for 2 to 3 hours, to outgas moisture and other volatile species trapped in the chamber during the wet clean process. Thereafter, the etch process to be performed in the chamber, is run for 10 to 15 minutes on dummy wafers, or until the chamber provides consistent and reproducible etching properties.
In the competitive semiconductor industry, the increased cost per substrate that results from the downtime of the etching chamber, during the dry or wet cleaning, and seasoning process steps, is highly undesirable. It typically takes 5 to 10 minutes for each dry cleaning process step, and 2 to 3 hours to complete the wet cleaning processes. Also, the wet cleaning and seasoning process often provide inconsistent and variable etch properties. In particular, because the wet cleaning process is manually performed by an operator, it often varies from one session to another, resulting in variations in chamber surface properties and low reproducibility of etching processes. Thus it is desirable to have an etching process that can remove or eliminate deposition of etch residue on the chamber surfaces.
In semiconductor fabrication, yet another type of problem arises in the etching of multiple layers of materials that have similar constituent elements, for example, silicon-containing materials such as tungsten silicide, polysilicon, silicon nitride, and silicon dioxide. With reference to FIGS. 1a and 1b, a typical polycide structure on a semiconductor substrate 25 comprises metal silicide 22 deposited over doped or undoped polysilicon 24. The polycide structures are formed over silicon dioxide 26, and etched to form the etched features 30. In these structures, it is difficult to obtain a high etching selectivity ratio for etching the metal silicide 22 relative to overlying resist 28, or the underlying polysilicon 24. It is especially desirable to have high etching selectivity ratios for polycide structures that have a convoluted topography, having thicker and thinner portions of metal silicide 22. This requires that the polysilicon 24 be etched sufficiently slowly relative to the rate of etching of the metal silicide 22, that all the polysilicon 24 below the thinner portions of metal silicide 22 are not etched through, before completion of etching of the thicker portion of overlying metal silicide 22. Thus, it is desirable to etch the metal silicide 22 at a faster rate relative to the rate of etching of the polysilicon 24.
A similar problem arises in the opening or etching of a mask layer of silicon nitride 32, on a thin silicon dioxide layer 34, prior to forming trenches in a substrate comprising silicon 36, as for example shown in FIGS. 1c, 1d and 1e. In this silicon trench isolation (STI) process, the nitride mask 32 is opened 38 to allow for the creation of etched trenches in the silicon 36 that are used, for example, to isolate active MOSFET devices formed on the substrate. The etching selectivity ratio for etching silicon nitride layer 32 relative to silicon dioxide 34 and the underlying silicon 36 has to be very high to stop on the silicon dioxide layer without etching through the silicon dioxide layer 34 and into the silicon substrate 36. FIG. 1d is a generalized depiction of the prior art process. In actuality the etch is not as ideal as shown in FIG. 1d. In fact, generally, the underlying silicon substrate 36 is somewhat etched into and the nitride layer 32 and the silicon dioxide layer 34 is not entirely etched. Maximizing the complete etch of the nitride layer 32 and the silicon dioxide layer 34 while minimizing the etching of the silicon substrate 36 is difficult. The nitride layer 32 must be etched in such a manner as to be highly selective to the oxide layer 34, often termed the pad oxide. Prior techniques have often etched through the oxide layer 34 and then detrimentally into the silicon substrate 36. In addition, poor selectivity control can result in the formation of projecting feet 40 of nitride and/or oxide, as shown in FIG. 1e. Etching through the oxide layer during the mask open process can lead to isotropic etching of the silicon substrate 36.
Conventional shallow trench isolation (STI) processes, in particular the hard mask open processes, result in large amounts of etchant residue being deposited on the substrate and on the chamber walls and surfaces. Typical STI processes require two separate process chambers. One chamber performs the nitride mask open process and another chamber is used for the shallow trench isolation (STI) silicon etch. Due to the large amounts of deposition residue that is generated, a wet cleaning of the substrate between the mask open and the STI etch must be performed. The chambers must also be frequently dry cleaned and wet cleaned.
High etching selectivity ratios are obtained by using process gas compositions that etch the different silicon-containing materials at significantly different etching rates, which depend upon the chemical reactivity of the particular process gas composition with a particular layer. However, etching metal silicide layers with high selectivity to polysilicon, or etching silicon nitride layers with high selectivity to silicon dioxide layers and/or silicon layers, is particularly difficult because all of the layers contain elemental silicon and most conventional etchant plasmas etch the silicon containing layers to form gaseous SiClx or SiFx species. Thus, it is difficult for the etchant plasma to chemically distinguish and preferentially etch the metal silicide layer 22 faster than the polysilicon layer 24, and the silicon nitride layer 32 faster than the silicon dioxide layer 34 or silicon 36. This problem is further exacerbated because the etchant residue formed on the chamber sidewalls also contains silicon dioxide, and attempts to remove the etchant residue during the polycide etching process, result in substantially lowering the rate of etching selectivity ratio of these layers.
Thus it is desirable to have an etch process that reduces formation of etch residue deposits in the etching chamber while maintaining a high etch selectivity.
In a method of etching a substrate, the substrate is placed in a chamber, and in a first stage, an energized first process gas comprising SF6 and Ar is provided in the chamber. The volumetric flow ratio of SF6 to other components of the first process gas is from about 5:1 to about 1:10. In a second stage, an energized second process gas comprising CF4 and Ar is provided in the chamber. The volumetric flow ratio of CF4 to other components of the second process gas is from about 1:0 to about 1:10.